Micrometer and nanometer scale structures are used in many fields including biological sciences, microelectromechanical systems (MEMS) and semiconductor manufacturing. For example, semiconductor devices such as microprocessors can be made up of millions of transistors, each interconnected by thin metallic lines branching on several levels and electrically isolated from each other by layers of insulating materials. Biological sensors may include microscopic regions of biological material that detect an analyte, transducers and electronics that provide an interpretable detectable signal.
When a new nanoscopic device is first produced in a fabrication facility, the design typically does not operate exactly as expected. It is then necessary for the engineers who designed the device to review their design and “rewire” it to achieve the desired functionality. Due to the complexity of lithography processes typically used to fabricate microstructures, it typically takes weeks or months to have the re-designed device produced. Further, the changes implemented frequently do not solve the problem, or the changes expose another flaw in the design requiring additional design changes. The process of testing, re-designing and re-fabrication can significantly lengthen the time to market new semiconductor devices. Device editing—the process of modifying a device during its development without having to remanufacture the whole circuit—provides tremendous economic benefits by reducing both processing costs and development cycle times.
Charged particle beam systems, such as focused ion beam systems and electron beam systems, are used to create and alter microscopic structures because the charged particles can be focused to a spot smaller than one tenth of a micron. Focused ion beams can micro-machine material by sputtering that is, physically knocking atoms or molecules from the target surface or chemically assisted ion beam etching. Electron beams can be used in chemically-assisted electron beam etching.
Ion beams, electron beams, and laser beams can also be used to directly deposit material by a process known as beam-induced deposition or direct-write deposition. Direct write deposition allows a device designer to test variations of the device without undertaking the lengthy process of modifying photolithography masks and fabricating a new circuit from scratch. Direct write deposition can be achieved by using electron beam, ion beam, or laser beam stimulated chemical vapor deposition, in which a precursor species dissociates due to the effects of the beam. Part of the dissociated molecules is deposited onto the substrate, and part of the dissociated molecule forms volatile by-products, which eventually release from the substrate surface. The precursor can be a vapor that contains a metal species to be deposited. The metal is deposited only in the area impacted by the beam, so the shape of the deposited metal can be precisely controlled. An ion beam assisted deposition process is described, for example, in U.S. Pat. No. 4,876,112 to Kaito et al. for a “Process for Forming Metallic Patterned Film” and U.S. Pat. No. 5,104,684 to Tao et al. for “Ion Beam Induced Deposition of Metals.”
It is often difficult to obtain high purity materials using direct write deposition, primarily due to the incorporation into the deposit of other components of the precursor molecules or the elements from the incident ion beam, such as gallium ions. This lack of control of composition, material purity, or internal structure often leads to undesirable properties in the deposited material. Tungsten and platinum deposited by focused ion beam (FIB)-induced deposition typically have resistivities greater than about 150 micro ohm centimeters (μΩ-cm). Recently-introduced FIB copper depositions have resistivities of 30-50 μΩ-cm. This is significantly higher than the resistivity of pure copper, which is less than 5 μΩ-cm. As conductor sizes continue to shrink and processor speeds increase, it will be necessary to reduce the resistivity of conductors deposited during the device edit process, so that the smaller conductors can carry the required current. Similarly, the resistivity of material used to fill vias, metal filled holes that connect conductors in different layers, will need to decrease because the diameter of vias will decrease in the future so there is less conductive material in the hole to carry current. Low resistivity of the fill material and elimination of voids thus becomes even more important. Also, as via dimensions decrease, it becomes more difficult to cleanly sever a line at the bottom of the via without redepositing conductive material on the sidewalls of the via, which can short circuit other layers.
Furthermore, the materials that can be deposited by charged particle beam-induced deposition are limited by the availability of vapor phase precursors with requisite properties, that is, high residency time (stickiness) on the surface, lack of spontaneous decomposition, and decomposition in the presence of the beam to deposit the desired material and form a volatile byproduct. When suitable deposition precursors do exist for a particular material, the deposition rates are often limited by gas depletion effects and other factors.
Processes for applying metal globally to a circuit are known. For example, copper electroplating has been used by IC manufactures to make on-chip interconnection in the Damascene process, originally developed by IBM in 1997. The electroplating bath solutions are specially formulated by and commercially available from various semiconductor chemical supplier companies. The IC manufacturing electroplating technology, known as superfilling during chip manufacturing has the capability of filling vias having diameters of about 100 nm with a 1:5 aspect-ratio. Such processes, however, are applied globally to an entire chip.
U.S. Pat. No. 7,674,706 to Gu et al. for “System for Modifying Structures Using Localized Charge Transfer Mechanism to Remove or Deposit Material” (“Gu”) describes depositing a localized drop of electrolyte on a portion of an integrated circuit and depositing or etching using an electric current flowing from a probe in the drop, through the electrolyte and then through the substrate. In one embodiment, the probe in the drop is replaced by using a charged particle beam to supply current, with the circuit being completed through the substrate.
FIG. 1 shows a method of localized electrochemical deposition of conductors using a micro or nano pipette in close proximity to a conductive surface. Such a method is described in Suryavanshi et al. in “Probe-based electrochemical fabrication of freestanding Cu nanowire array,” Applied Physics Letters 88, 083103 (2006) (“Suryavanshi”). A glass pipette 102 holds an electrolyte solution 104, such as 0.05 M CuSO4. A power supply 106 provides current for the electrochemical reaction, with an electric circuit being formed between a copper electrode 108 and a conductive substrate 110. The process is typically carried out in atmosphere under the observation of an optical microscope. A device that moves about a surface writing a pattern is referred to as a “nano pen.”